Retain focused node when redrawing tokenizer/typeahead results

Summary:
Ref T4420. Fixes T5473. Currently, when typeahead results get redrawn, you can lose your cursor position. A simple way to reproduce this is type "dif", select "Differential" using the arrow keys, then type "f". The selection will be lost.

Instead: store the old selection, then look for an item with the same name in the new set and select it. In effect, this preserves any focus selection.

Test Plan:

• Typed "dif".
• Typed "down arrow key" to select "Differential".
• Typed "f".
• "Differential" remained selected.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5473, T4420

Differential Revision: https://secure.phabricator.com/D9900